The invention relates to a process for co-production of paraxylene and metaxylene that comprises in combination 1) a unit for isomerization of the C8-aromatic compounds that dealkylate ethylbenzene, 2) an adsorption unit of a simulated moving bed that has the particular feature of comprising three effluents: an extract that consists of paraxylene and desorbent, an intermediate fraction (extract or raffinate) that contains ethylbenzene with a yield that is close to 100%, a raffinate that contains a mixture of metaxylene and orthoxylene that is substantially free of ethylbenzene and paraxylene, 3) a separation by distillation of orthoxylene from the mixture of metaxylene and orthoxylene.
The production of high-purity paraxylene by separation by adsorption in a simulated moving bed is well known from the prior art. This market is extensively developed; its outlets are the productions of terephthalic acid, phthalic anhydride and terephthalate polyethylene resins. In contrast, the metaxylene market is still restricted, whereby its outlet is isophthalic acid. It was recently perceived that the addition of small amounts of isophthalate polyethylene to terephthalate polyethylene improved the properties of the latter. It therefore becomes advantageous to co-produce paraxylene and metaxylene in the same aromatic compound production complex provided that the market requirements are satisfied: the amount of paraxylene that is produced should be much larger than that of metaxylene: typically 5 to 40 times larger, the paraxylene should be very pure, typically at least 99.6%, and the metaxylene should have reasonable purity, typically at least 99.0%.
The technological background that describes the production of paraxylene with very high purity is illustrated in patent E-PA-531 191 of the applicant.
The prior art knows metaxylene production processes, for example, U.S. Pat No. 4,326,092 where the adsorbent is a zeolite Y of molar ratio Si/Al on the order of 4.5 to 5 exchanged with sodium and where the separation is carried out by the technique of adsorption in a simulated moving bed in liquid phase. In U.S. Pat. No. 5,382,747, the same separation is carried out on a zeolite Y that is exchanged with lithium and sodium, in a restricted range of temperature and degree of hydration by using the toluene as a desorbent. To co-produce the paraxylene in the great majority and metaxylene, the drawback of these processes is to require two separate units of very different sizes, without a possibility of finding any synergy in the co-production of the two isomers.
The prior art also describes processes of co-production of paraxylene and metaxylene; for example, U.S. Pat. No. 4,368,347 uses a vapor phase process with intermediate fraction recycling: in addition to the complication that is linked to recycling of intermediate fractions, this document does not suggest how it is possible to use in a practical way such a process that operates at a pressure of between 1 and 2 bar and at a temperature of 150 to 200xc2x0 C. with a feedstock whose bubble point is 145xc2x0 C. and with fixed beds that have pressure drops of at least 0.1 bar and probably more to operate economically. Patent FR 2 651 148 uses two different solvents to separate the C8-aromatic fraction into three effluents, which greatly limits its scope since the distillations that result from the simulated moving bed separation unit are multiplied. Patent WO 93/22022 describes various cases of separations of feedstocks of three components into three effluents, however, the technology. that is used that involves both very high pressures, a pressure regulation and a flow rate regulation in each of the three or four zones of the process and beds that are each separated in a column is justified economically only for products of high added value.
U.S. Pat. No. 4,306,107 describes a simulated moving bed process in liquid phase where the metaxylene is sampled in the form of extract; the paraxylene, orthoxylene and a fraction of ethylbenzene are sampled as an intermediate raffinate; and finally the ethylbenzene is sampled as a raffinate. This process naturally does not allow a majority of paraxylene and an accompanying stream of metaxylene to be co-produced.
The document of the prior art which comes closest to the invention is U.S. Pat. No. 4,313,015; this document describes the separation of a simulated moving bed, in liquid phase, on zeolite X that is exchanged with barium, whereby the desorbent is diethylbenzene. The extract consists of paraxylene that is too impure (99.44%) to be marketed at current standards (current standard=99.6 mini) and with a yield of 97.5%; the intermediate raffinate consists of ethylbenzene, metaxylene and orthoxylene and a little paraxylene; finally the raffinate consists primarily of a mixture of orthoxylene and metaxylene, whereby the metaxylene can be separated by distillation. The text specifies that the intermediate raffinate is sampled approximately in the center of the zone between the introduction point of the feedstock and the sampling point of the raffinate. The feedstock that is dealt with in the example is not completely representative of a feedstock that is found in a refinery: the latter always contain at least traces and sometimes up to 5% paraffins and naphthenes with eight and nine carbon atoms, which distill in the same temperature interval as xylenes. This document does not specify how the paraffins and naphthenes separate between the raffinate and the intermediate raffinate, the total number of beds and the number of beds per zone used, the order of magnitude of the flow rates in each of the zones of the process and more particularly the one in zone 1, therefore the necessary solvent level and the exchange time of the beds (circulation speed of the solid).
The object of the invention is the co-production of paraxylene and metaxylene that can be marketed from a hydrocarbon feedstock that can actually be produced in a refinery. A second object of the invention is to obtain paraxylene with a purity of at least 99.6%, with a minimum yield of 98% and metaxylene with a purity that is at least equal to 99% after distillation. A third object of the invention is to produce, from the same separation unit, substantially more paraxylene than metaxylene (for example at least twice more, for example five times more).
More specifically, the invention relates to a process for co-production of paraxylene and metaxylene from a hydrocarbon feedstock that comprises them, whereby the process comprises a separation stage of said mixture in a simulated moving bed in countercurrent or in co-current in at least one chromatographic column that contains a number of beds of an adsorbent that are interconnected in a closed loop and that have a selectivity that is different for paraxylene, ethylbenzene, metaxylene and orthoxylene, whereby said column comprises at least five zones that are delimited by injections of a feedstock and a desorbent and draw-offs of an intermediate raffinate, a raffinate and an extract, whereby a paraxylene desorption zone 1 is between the injection of the desorbent and the sampling of the extract; a zone 2 for desorption of ethylbenzene, orthoxylene and metaxylene is between the sampling of the extract and the injection of the feedstock; a paraxylene adsorption zone 3A is between the injection of the feedstock and the draw-off of intermediate raffinate; an ethylbenzene adsorption zone 3B is between the draw-off of intermediate raffinate and the draw-off of raffinate; a zone 4 is between the raffinate draw-off and the injection of desorbent, whereby the process is characterized in that:
the feedstock has an ethylbenzene content that is less than 5% by weight,
the chromatographic column comprises at least twenty-five beds, of which at least five beds are in zone 3B,
the raffinate is distilled at least once to recover metaxylene with a purity of at least 99.0% and orthoxylene and
the extract is distilled at least once to recover paraxylene with a purity of at least 99.6%.
Thus, from a complex that comprises a single adsorption unit, with a very high purity, a major ratio of paraxylene and a minor ratio of metaxylene are produced, whereby the metaxylene ratio is obtained with a yield of at least 50%.
In addition, to obtain an economical ortho-metaxylene distillation, it is possible to allow a significant metaxylene fraction to go from the bottom of the distilling column. The recovery level by metaxylene distillation can be at least 50%.
According to a characteristic of the process, the feedstock advantageously can have an ethylbenzene content of less than 2.5% by weight, preferably less than 1.25%. It can also have a content of linear, branched and cyclic alkanes that is less than 1% by weight and advantageously less than 0.1%.
According to a preferred characteristic of the invention, it is possible to inject desorbent into zone 1 and the feedstock into zone 3A of the column in a ratio by. weight of desorbent to feedstock of at least 1.8:1, preferably at least 1.9:1.
When this solvent level of at least 1.9:1 is combined with a total number of beds in the column of at least 30 beds, including at least 6 beds in zone 3b, excellent results in terms of purity both in the paraxylene (greater than 99.7) and in the metaxylene (greater than 99.5) and in terms of yield (greater than 98.5 for paraxylene) are obtained.
According to another characteristic, the ratio of flow rates (without desorbent) of raffinate to intermediate raffinate that are drawn off can be less than 0.5 and preferably less than 0.3. As has been described above, the feedstock generally contains less than 5% by weight of ethylbenzene. It can therefore come from either a unit for transalkylation of C7 and C9 into xylenes or a unit for catalytic dismutation of toluene into benzene and xylenes or an isomerization unit of a fluid that contains ethylbenzene, whereby said unit is operated in the presence of a catalyst that dealkylates ethylbenzene into benzene as described in U.S. Pat. No. 5,516,956 and WO 98/05 613 that are incorporated as references, whereby the isomerate that is obtained can be recycled in the unit if it is not adequately dealkylated at the end of a first pass.
After distillation of the desorbent, the intermediate raffinate that contains metaxylene, orthoxylene, ethylbenzene and praraxylene is isomerized in the presence of a preferably dealkylating isomerization catalyst, and the isomerate is recycled at least in part in the adsorption column of a simulated moving bed at the feedstock.
The raffinate that is drawn off at the end of zone 3B contains metaxylene and orthoxylene as well as desorbent. The latter is distilled, and the mixture that contains orthoxylene, once separated by a suitable distillation, is at least in part isomerized as described above and the isomerate that is recycled at least in part at the injection point of the feedstock in the adsorption column.
The recovered desorbent is generally recycled at least in part in the adsorption column at the injection point of the desorbent.